Optical semiconductor device

ABSTRACT

The present invention is intended to provide a compact and simple optical semiconductor device that reduces crosstalk (leakage current) between light receiving elements. According to the present invention, since a back surface electrode is a mirror-like thin film, crosstalk to an adjacent light receiving element can be suppressed, thereby reducing a detection error of a light intensity. By disposing a patterned back surface electrode or by disposing an ohmic electrode at the bottom of an insulating film over the whole back surface, contact resistance on the back surface can be reduced. By using the optical semiconductor elements with a two-dimensional arrangement and by using a mirror-like thin film as the back surface electrode, crosstalk can be reduced. By accommodating the optical semiconductor elements in the housing in a highly hermetic condition, the optical semiconductor elements can be protected from an external environment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No.2011-015580 filed Jan. 27, 2011, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an optical semiconductordevice that is applied to optical fiber communications, and specificallyrelates to an array of optical semiconductor light receiving elements aslight receiving elements (photodiodes: PDs) that can adapt tomulti-channelization.

2. Description of the Related Art

With the recent development of an optical fiber communication technologysuch as a multiple wavelength communication, a light receiving elementthat can detect light having more channels is demanded. Meanwhile, inorder to prevent increase of size of a device with multi-channelization,downsizing and integration of a device is also demanded. To fulfillthese demands, an optical semiconductor device on which light receivingelements in an array are formed is widely used since it can receivelight of multi-channel and is compact.

FIG. 1A is an external view of a conventional optical semiconductordevice described in Japanese patent Laid-Open No. 2007-266251. FIG. 1Bis a cross-sectional view of the conventional optical semiconductordevice, the cross-sectional view including a light receiving section.FIG. 1A illustrates, as an example, an array of optical semiconductorelements, each having a light receiving section, the array beingcomposed of four elements. The number of the elements can be increasedor decreased according to application.

The optical semiconductor device in FIGS. 1A and 1B has a lightabsorbing layer 112 formed on a conductive semiconductor substrate 110,and a plurality of diffusion regions 120 that has a conductive propertyopposite to that of the conductive semiconductor substrate 110. Thelight absorbing layer 112 has an insulation property. In such aconfiguration, immediately on the light absorbing layer 112, aconductive semiconductor layer 114 is disposed, and the diffusionregions 120 are formed in the conductive semiconductor layer 114. On thesemiconductor substrate 110, a back surface electrode 118 is formed by,for example, evaporation, and on the conductive semiconductor layer 114,an insulating film 116 and a front surface electrode 119 are formed. Inthis device, an optical semiconductor element 100 is mounted in such away that the back surface electrode 118 is fixed with the use of metalsolder 130, and the front surface electrode 119 is connected to anelectrical wiring 136 formed on an electrical wiring board 134 with theuse of a bonding wire 132.

As a material of the optical semiconductor element 100, silicon (Si),germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP) or thelike is used. Hereinafter, the optical semiconductor element using anInP-based material, which is widely used for long-distance optical fibercommunications, will be described.

The conductive semiconductor substrate 110 is made of n-type InP(carrier concentration: 1×10¹⁸ cm⁻³), the light absorbing layer 112 ismade of insulating (n-type) indium gallium arsenide (InGaAs, carrierconcentration: 1×10¹⁴ cm⁻³), the conductive semiconductor layer 114 ismade of n-type InP (carrier concentration: 1×10¹⁷ cm⁻³), and thediffusion regions 120 formed in the conductive semiconductor layer 114are made of Zn-doped p-type InP (carrier concentration: 1×10¹⁸ cm⁻³).For the insulating film 116 formed on the conductive semiconductor layer114, silicon nitride (SiN) is used. The insulating film 116 has apassivation function for semiconductor junction, and also serves as ananti-reflective coating when light enters.

A light receiving diameter of a light receiving section 140 is 80 μm; aninterval between light receiving elements is 250 μm; and a thickness ofthe conductive semiconductor substrate 110 is about 200 μm.

In order that the back surface electrode 118 effectively functions as acommon cathode of the array of light receiving elements, an ohmicelectrode is commonly disposed. That is, an alloy is inserted forreducing a Schottky barrier at an interface between the InP substrate110 and the metal solder 130. Since this conventional example uses ann-type substrate, an alloy of germanium-containing gold and nickel isused. The alloy is deposited on the InP substrate by evaporation, andafter that gold and germanium are diffused into InP by heat treatment,thereby reducing the Schottky barrier and making the interface ohmic.Although not illustrated in FIG. 1, on the bottom of the ohmic electrode118, an electrode that contains titanium, platinum, gold or the like maybe further added.

Operation of the optical semiconductor device illustrated in FIGS. 1Aand 1B will be described. First, a reverse bias voltage is appliedbetween the front surface electrode 119 and the back surface electrode118. As illustrated in FIG. 1B, most of incident light 150 inputted intothe light receiving section 140 thorough the insulating film 116 fromthe surface is photoelectrically converted into both carriers ofelectrons 171 and holes 172 in the light absorbing layer 112. In thelight absorbing layer 112 (insulating InGaAs) that was depleted by thereverse bias voltage, a gradient of an energy band occurs. Accordingly,each of the carriers, electrons 171 and holes 172 generated in the lightabsorbing layer 112 move by drift to the semiconductor substrate 110(n-type InP) and the p-type diffusion regions 120 (p-type InP),respectively, and are finally emitted outside from the electrodes formedon the front and back surfaces.

Part of the incident light 150 inputted to the light absorbing layer 112is not completely photoelectrically converted in the light absorbinglayer 112, and becomes a substrate-transmitting light 152. Thesubstrate-transmitting light 152 is reflected by the back surfaceelectrode 118 and part of the reflected light may be inputted into thelight absorbing layer 112 again, but some of the light reaches anadjacent element 162, as indicated by a dashed line arrow 154 in FIG.1B.

SUMMARY OF THE INVENTION

In the conventional optical semiconductor device illustrated in FIG. 1,there is a problem that when part of the substrate-transmitting light152 reflected by the back surface electrode 118 reaches the adjacentelement 162 and the light absorbing layer 112 near the adjacent element162 as described above, crosstalk (leakage current) 170 occurs in theadjacent element 162. The occurrence of the leakage current 170 causes aproblem that a detection error of a light intensity may occurs inmonitoring a light intensity in optical fiber communications.

As a result of analyzing factors of such crosstalk, the following threephenomena are estimated to be main factors: (1) light crosstalk thatreaches an adjacent element due to diffuse reflection by a back surfaceof a substrate (2) electrical crosstalk due to diffusion of carriers(electrons and holes) generated by light that reaches an absorbing layernear an adjacent element, (3) crosstalk due to some phenomena includingthe (1) and (2) phenomena that are exercised by substrate-transmittinglight that was not completely photoelectrically converted in a lightabsorbing layer of an input element.

Prior literatures disclose measures against the phenomena (2) and (3).

First, with respect to the phenomenon (2), Japanese patent Laid-Open No.2007-266251 discloses a structure in which a second semiconductorjunction layer is provided between light receiving elements.Accordingly, carriers that are generated in an absorbing layer near anadjacent element can be extracted by drift, thereby reducing thecrosstalk.

Next, with respect to the phenomenon (3), by a method such as thickeningthe light absorbing layer or providing a plurality of light absorbinglayers, the substrate-transmitting light can be reduced, therebyreducing the crosstalk. However, the substrate-transmitting light cannotbe completely suppressed. Therefore, it is an important problem toreduce the crosstalk due to the diffuse reflection at the back surfacein the phenomenon (1).

However, the prior literature does not disclose a technique for reducingthe diffuse reflection at the back surface in the phenomenon (1).

A cause to generate the diffuse reflection will be described with theuse of FIG. 1B. On the bottom of the conductive semiconductor substrate110, the ohmic electrode is disposed as the back surface electrode 118.In such an ohmic electrode, gold and germanium are diffused into InP byheat treatment as described above, which roughens the interface betweenInP and the ohmic alloy. FIG. 1B schematically illustrates the roughinterface between the back surface electrode 118 and InP. This roughnessof the back surface causes the diffuse reflection, thereby causing theproblem of generating the crosstalk to an adjacent element. The presentinvention has been made in view of such a problem and is intended toprovide a compact and simple optical semiconductor device that cansufficiently reduce a leakage current between light receiving elements.

The present invention provides an optical semiconductor device thatincludes a conductive semiconductor substrate, a light absorbing layerformed on the conductive semiconductor substrate, and a conductivesemiconductor layer formed on the light absorbing layer, in which theconductive semiconductor layer has a plurality of diffusion layers thathave a conductivity property opposite to that of the conductivesemiconductor substrate thereby to form light receiving elements in anarray and the bottom of the conductive semiconductor substrate isprovided with a mirror-like thin film.

In one embodiment of the present invention, the mirror-like thin filmincludes a back surface electrode containing barrier metal.

In one embodiment of the present invention, the mirror-like thin film ispatterned.

In one embodiment of the present invention, the mirror-like thin film ispatterned, and the optical semiconductor device has a second backsurface electrode formed on the bottom of the mirror-like thin film, thesecond back surface electrode being an ohmic electrode.

In one embodiment of the present invention, the mirror-like thin filmincludes an insulating film.

In one embodiment of the present invention, the mirror-like thin filmincludes an insulating film and a back surface electrode at the bottomof the insulating film.

In one embodiment of the present invention, the mirror-like thin filmincludes an insulating film and a back surface electrode at the bottomof the insulating film, and is patterned.

In one embodiment of the present invention, the mirror-like thin filmincludes an insulating film and a first back surface electrode at thebottom of the insulating film, and is patterned, and the semiconductordevice has a second back surface electrode formed at the bottom of themirror-like thin film, the second back surface electrode being an ohmicelectrode.

In one embodiment of the present invention, the optical semiconductordevice is accommodated in a housing.

In one embodiment of the present invention, the light receiving elementsare two-dimensionally arranged.

In the semiconductor device according to the present invention, byemploying the mirror-like thin film as the back surface electrode, aleakage current to an adjacent light receiving element can be easilysuppressed, thereby reducing a detection error of a light intensity inan optical semiconductor device.

Further, by disposing the patterned back surface electrode or the ohmicelectrode on the bottom of the insulating film over the whole backsurface, a contact resistance on the back surface can be reduced.

Further, by using the two-dimensionally arranged optical semiconductorelements and by using the mirror-like thin film as the back surfaceelectrode, crosstalk can be reduced.

By accommodating the optical semiconductor element in the housing in ahighly hermetic condition, the optical semiconductor element can beprotected from an external environment, be excellent in humidityresistance and have high reliability.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views for explaining a structure of a conventionaloptical semiconductor device, in which FIG. 1A is an external view andFIG. 1B is a cross sectional view;

FIGS. 2A and 2B are views for explaining a structure of an opticalsemiconductor device according to a first embodiment of the presentinvention, in which FIG. 2A is a cross sectional view and FIG. 2B is adetailed view of a back surface structure;

FIG. 3 is a view for explaining a structure of an optical semiconductorelement according to the first embodiment of the present invention, inwhich the optical semiconductor element is accommodated in a housing;

FIG. 4 is a graph showing an evaluation result of the semiconductorelement according to the first embodiment of the present invention;

FIGS. 5A and 5B are views for explaining a structure of an opticalsemiconductor device according to a second embodiment of the presentinvention, in which FIG. 5A is a cross sectional view and FIG. 5B is abottom view;

FIGS. 6A and 6B are views for explaining a structure of an opticalsemiconductor device according to a variation of the second embodimentof the present invention, in which FIG. 6A is a cross sectional view andFIG. 6B is a bottom view;

FIGS. 7A and 7B are views for explaining a structure of an opticalsemiconductor device according to a third embodiment of the presentinvention, in which FIG. 7A is a cross sectional view and FIG. 7B is abottom view;

FIGS. 8A and 8B are views for explaining a structure of an opticalsemiconductor device according to a variation of the third embodiment ofthe present invention, in which FIG. 8A is a cross sectional view andFIG. 8B is a bottom view;

FIG. 9 is a cross sectional view for explaining a structure of anoptical semiconductor device according to a fourth embodiment of thepresent invention;

FIG. 10 is a cross sectional view for explaining a structure of anoptical semiconductor device according to a variation of the fourthembodiment of the present invention;

FIGS. 11A and 11B are views for explaining a structure of an opticalsemiconductor device according to a fifth embodiment of the presentinvention, in which FIG. 11A is a cross sectional view and FIG. 11B is abottom view;

FIGS. 12A and 12B are views for explaining a structure of an opticalsemiconductor device according to a variation of the fifth embodiment ofthe present invention, in which FIG. 12A is a cross sectional view andFIG. 12B is a bottom view;

FIGS. 13A and 13B are views for explaining a structure of an opticalsemiconductor device according to a sixth embodiment of the presentinvention, in which FIG. 13A is a cross sectional view and FIG. 13B is abottom view;

FIGS. 14A and 14B are views for explaining a structure of an opticalsemiconductor device according to a variation of the sixth embodiment ofthe present invention, in which FIG. 14A is a cross sectional view andFIG. 14B is a bottom view;

FIG. 15 is a perspective view for explaining a structure of an opticalsemiconductor device according to a seventh embodiment of the presentinvention;

FIG. 16 is a view for explaining a structure of the opticalsemiconductor element according to the seventh embodiment of the presentinvention, in which the optical semiconductor element is accommodated ina housing; and

FIG. 17 is a graph showing an evaluation result of the semiconductorelement according to the seventh embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

An optical semiconductor device according to a first em embodiment ofthe present invention will be described with reference to FIGS. 2 to 4.FIGS. 2A and 2B are views illustrating a configuration of the opticalsemiconductor device, in which FIG. 2A is a cross sectional viewincluding a light receiving section and FIG. 2B is a detailed view of aback surface structure. FIG. 2A illustrates, as an example, an array ofoptical semiconductor elements that include a plurality of lightreceiving sections, but the number of the elements may be increased ordecreased according to application.

The optical semiconductor device illustrated in FIG. 2 has a lightabsorbing layer 112 formed on a conductive semiconductor substrate 110and a plurality of conductive diffusion regions 120 that have aconductive property opposite to that of the conductive semiconductorsubstrate 110. This light absorbing layer 112 has an insulationproperty. In this configuration, a conductive semiconductor layer 114 isdisposed immediately on the light absorbing layer 112, and the diffusionregions 120 are formed in the conductive semiconductor layer 114. On thesemiconductor substrate 110, a back surface electrode 118 is formed byevaporation or the like, and on the conductive semiconductor layer 114,an insulating film 116 and a front surface electrode 119 are formed.

As a material constituting the optical semiconductor element, silicon(Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP) orthe like is used. In this embodiment, the optical semiconductor elementmade of an InP-based material, which is widely used for a long-distanceoptical fiber communication, will be described, as with a conventionalexample.

The conductive semiconductor substrate 110 is made of n-type InP(carrier concentration: 1×10¹⁸ cm⁻³); the light absorbing layer 112 ismade of insulating (n-type) indium gallium arsenide (InGaAs, carrierconcentration: 1×10¹⁴ cm⁻³); the conductive semiconductor layer 114 ismade of n-type InP (carrier concentration: 1×10¹⁷ cm⁻³); and thediffusion regions 120 formed in the conductive semiconductor layer 114are made of Zn-doped p-type InP (carrier concentration: 1×10¹⁸ cm⁻³).For the insulating film 116 formed on the conductive semiconductor layer114, silicon nitride (SiN) is used. This insulating film 116 has apassivation function for semiconductor junction, and also serves as ananti-reflective coating when light enters.

A light receiving diameter of the light receiving section 140 is 80 μm,an interval between light receiving elements is 250 μm, and a thicknessof the conductive semiconductor substrate 110 is about 200 μm.

A structural difference from the cross-sectional view of theconventional example in FIG. 1B is a structure of the back surfaceelectrode 118. That is, in the present invention, the back surface ofthe semiconductor substrate 110 is provided with, as the back surfaceelectrode 118, a mirror-like thin film, instead of ohmic metal made ofan alloy. A material of the mirror-like thin film may be selected fromvarious materials such as metals and insulating films. In the presentembodiment, metal that contains barrier metal is used. The barrier metalis metal that is inserted between diffusible materials in order toprevent interdiffusion between the materials. Since gold wiring andgold-tin solder are used in the present embodiment, platinum is used asthe barrier metal for preventing diffusion of gold and InP.

Details of a back surface structure will be described with reference toFIG. 2B. First, titanium 1182, which has good adhesion with InP, isattached by evaporation onto the bottom of the n-type InP that is theconductive semiconductor substrate 110. Then, platinum 1184, which isthe barrier metal, is attached onto the bottom of the titanium 1182 byevaporation. Further, gold 1186 is attached onto the bottom of theplatinum 1184 by evaporation. A thickness of each of the layers is about500 Å. In such a structure, interdiffusion between InP and gold isunlikely to occur during heating process or the like by inserting thebarrier metal between them. Accordingly, it is observed under an SEMmicroscope that an interface between InP and the back surface metalcomes into a flat and smooth state, and a good mirror-like thin film isformed.

Operation of the optical semiconductor device according to the presentembodiment will be described. First, a reverse bias voltage is appliedbetween the front surface electrode 119 and the back surface electrode118. As illustrated in FIG. 2A, most of incident light 150 inputted intothe light receiving section 140 through the insulating film 116 from thesurface is photoelectrically converted in the light absorbing layer 112into both carriers of electrons and holes. In the light absorbing layer112 (insulating InGaAs) that was depleted by the reverse bias voltage, agradient of an energy band occurs. Accordingly, each of the carriers,i.e., electrons and holes, generated in the light absorbing layer 112move by drift to the semiconductor substrate 110 (n-type InP) and thep-type diffusion regions 120 (p-type InP), respectively, and are finallyemitted outside from the electrodes formed on the front and backsurfaces.

Part of the incident light 150 inputted into the light absorbing layer112 is not completely photoelectrically converted in the light absorbinglayer 112 and becomes substrate-transmitting light 152. Thesubstrate-transmitting light 152 is reflected by the back surfaceelectrode 118. Since the back surface electrode 118 is a mirror-likethin film, it exhibits reflection operation like not diffuse reflectionbut specular reflection on the back surface. Therefore, light thatreaches an adjacent element by diffuse reflection on the back surface asin the conventional example can be reduced and reduction of crosstalkcan be expected.

The optical semiconductor element illustrated in FIG. 2 can beaccommodated in a housing 182 and a window lid 184 as illustrated inFIG. 3. The optical semiconductor element 100 is accommodated in thebox-shaped housing 182 made of ceramic, and is hermetically sealed withthis housing 182 and the window lid 184 made of sapphire or the likethat enables light to enter the light receiving section 140. Since thehousing 182 and the window lid 184 are bonded by the metal solder 130(not shown), the optical semiconductor element can be protected from anexternal environment, be excellent in humidity resistance and have highreliability in a highly hermetic condition. The optical semiconductorelement 100 is accommodated in such a way that the light receivingsection 140 faces the window lid 184, the back surface electrode 118 andthe housing 182 are fixed to each other by the metal solder 130 or thelike, and the front surface electrode 119 is connected to the electricalwiring 136 in the housing by the bonding wire 132. The electrical wiring136 in the housing 182 penetrates through the housing 182 to the surfaceof the housing 182 (not shown), allowing for an electrical connection toan electrical wiring board connected to outside.

FIG. 4 shows a result of evaluating an amount of crosstalk when lightwas inputted from an optical fiber. This evaluation was performed insuch a way that the optical semiconductor element 100 was mounted on thehousing 182 illustrated in FIG. 3, and light was inputted into the lightreceiving section 140 with the use of the optical fiber without sealingby the window lid 184. Measurement was performed with the use of inputlight having a wavelength of 1.55 μm in a room temperature environment.A crosstalk value of a vertical axis is a ratio between a lightreceiving current in a light-inputted element and a light receivingcurrent in an adjacent element. In doing measurement, by changing adistance (z) between the light receiving section and an end face of theoptical fiber, a change of crosstalk was observed.

The result of the above experiment shows that an adjacent crosstalkvalue is −35 dB to −42 dB in a region of z<600 μm in the conventionalelement whereas an adjacent crosstalk value is −45 dB to −52 dB in theelement of the present invention, causing the reduction in crosstalk by10 dB.

In the present embodiment, since an ohmic electrode is not used as theback surface electrode 118, the InP substrate 110 has a Schottky contactwith the back surface electrode 118, causing a concern of contactresistance. However, since the back surface electrode 118, which is acommon electrode, has a large attachment area, increase of contactresistance often does not matter in quality under a normal operatingcondition. For example, if the resistance increases by about 1Ω,degradation of a band is about 50/51, which is subtle, in a transmissionpath with a load resistance of 50Ω. In addition, a voltage drop due tocontact resistance is about 1 mV at a light receiving current of 1 mA,which is subtle and can be ignored.

In the present embodiment, the conductive semiconductor substrate 110 isn-type, but P-type conductive semiconductor substrate 110, which has anopposite conductive property, has the same improving effect. In such acase, the conductive semiconductor substrate 110 is p-type, the lightabsorbing layer 112 is p-type, the conductive semiconductor layer 114 isp-type, and the diffusion regions 120 formed in the conductivesemiconductor layer 114 are n-type.

In the present embodiment, since the back surface exhibits thereflection operation like specular reflection, an amount of thesubstrate-transmitting light 152 inputted to the light absorbing layer112 of the light-inputted element 160 increases, thereby exhibiting aneffect to increase light receiving sensitivity of the light-inputtedelement 160.

An optical semiconductor device according to a second embodiment of thepresent invention will be described with reference to FIGS. 5A and 5B.FIGS. 5A and 5B are views illustrating a configuration of the opticalsemiconductor device, in which FIG. 5A is a cross sectional viewincluding a light receiving section and FIG. 5B is a bottom view beforemetal-soldering.

As with the first embodiment, such an optical semiconductor device hasthe light absorbing layer 112 formed on the conductive semiconductorsubstrate 110 and has the plurality of diffusion regions 120 that have aconductive property opposite to that of the conductive semiconductorsubstrate 110. Description of configurations of the carrierconcentration, electrodes, and the like is left out since theconfigurations are the same as those of the first embodiment.

The present embodiment is structurally different from the firstembodiment in that, in the present embodiment, the back surfaceelectrode 118 is not a full-surface electrode but patterned. Thepatterning is performed by a lift-off process in which the bottom of theconductive semiconductor substrate 110 is coated with organic resist andexposed. A patterned position is on the side opposite to each of thelight receiving sections 140 on the surface of the light receivingelement through the InP substrate 110. That is, the patterned electrodeon the back surface is positioned so that an optical axis passingthrough each of the light receiving sections is in the center of thepatterned electrode. In the present embodiment, as illustrated in FIG.5B, the patterned electrode is circular and the diameter of theelectrode is φ=200 μm. In a region without the patterned electrode, theconductive semiconductor substrate 110 is uncovered.

The back surface electrode is formed by not using ohmic alloy metal butplacing a mirror-like thin film as with the first embodiment. In thepresent embodiment, as with the first embodiment, metal that containsbarrier metal is used. In the present embodiment, since gold wiring andgold-tin solder are used, platinum is used as the barrier metal forpreventing diffusion of gold and InP. On the bottom of the n-type InPthat is the conductive semiconductor substrate 110, titanium, platinumand gold are attached in this order by evaporation. A thickness of eachof the layers is about 500 Å.

Such a structure, as with the first embodiment, suppressesinterdiffusion between InP and gold, thereby forming a good mirror-likethin film. In addition, in the present embodiment, since the backsurface electrode is not disposed over the whole back surface, an areaof platinum, which tends to generate stress due to heat change, can bereduced. Therefore, a thin film can be formed that maintains amirror-like surface in spite of heat change and is unlikely to be peeledoff.

Operation of the optical semiconductor device according to the presentembodiment will be described. As with the first embodiment, first, areverse bias voltage is applied between the surface electrode 119 andthe back surface electrode 118. As illustrated in FIG. 5A, most of theincident light 150 inputted into the light receiving section 140 throughthe insulating film 116 from the surface is photoelectrically convertedin the light absorbing layer 112. However, part of the incident light150 inputted into the light absorbing layer 112 is not completelyphotoelectrically converted in the light absorbing layer 112 and becomesthe substrate-transmitting light 152, which is reflected by the backsurface electrode 118. Since the back surface electrode 118 is amirror-like thin film, the back surface exhibits reflection operationlike not diffuse reflection but specular reflection. Therefore, lightthat reaches an adjacent element by the diffuse reflection on the backsurface as in the conventional example can be reduced, thereby reducingcrosstalk.

In the present embodiment, since an ohmic electrode is not used as theback surface electrode 118, the InP substrate 110 has a Schottky contactwith the back surface electrode 118. In addition, the back surfaceelectrode 118 is patterned. Therefore, increase of contact resistance ismore concerned than in the first embodiment. However, as with the firstembodiment, since the back surface electrode 118, which is a commonelectrode, has a large attachment area, increase of contact resistanceoften does not matter in quality under a normal operating condition.

Next, a variation of the second embodiment will be described withreference to FIGS. 6A and 6B. FIGS. 6A and 6B are views illustrating aconfiguration of a semiconductor device, in which FIG. 6A is a crosssectional view including a light receiving section and FIG. 6B is abottom view.

This structure is different from that of the embodiment illustrated inFIGS. 5A and 5B with respect to a pattern shape of the back surface. Inthe present embodiment, in addition to the pattern (diameter: 200 μm)placed so that an optical axis passing through a light receiving sectionis in the center of the pattern, back surface electrodes, each having asmall diameter (diameter: 50 μm), are placed around the pattern.

In addition to effects of the invention in the embodiment illustrated inFIGS. 5A and 5B, in such a structure, tight adhesion between the backsurface electrodes having a small diameter and metal solder increasesadhesive strength between the optical semiconductor element and themetal solder, thereby enabling the optical semiconductor element tostrongly be fixed to the electric wiring and housing. A larger area ofthe back surface electrode can reduce contact resistance.

An optical semiconductor device according to a third embodiment of thepresent invention will be described with reference to FIGS. 7A and 7B.FIGS. 7A and 7B are views illustrating a configuration of the opticalsemiconductor device, in which FIG. 7A is a cross sectional viewincluding a light receiving section and FIG. 7B is a bottom view.

As with the above embodiments, such an optical semiconductor device hasthe light absorbing layer 112 formed on the conductive semiconductorsubstrate 110 and has the plurality of diffusion regions 120 that havean conductive property opposite to that of the conductive semiconductorsubstrate 110. Description of configurations of the carrierconcentration, electrodes, and the like is left out since theconfigurations are the same as those of the above embodiments.

The present embodiment is structurally different from the aboveembodiments in that in the present embodiment after an electrodecontaining barrier metal is disposed as a first back surface electrode1187 on the back surface, an ohmic electrode is further disposed as asecond back surface electrode 1188. In the present embodiment, the firstback surface electrode 1187 is not a full-surface electrode butpatterned, and on the bottom of the first back surface electrode 1187,the ohmic electrode is further disposed as the second back surfaceelectrode 1188 over the whole back surface. The method and structure ofthe patterning are the same as those of the second embodiment. That is,the pattern is created by a lift-off process and is positioned oppositeto each light receiving section 140 on the surface of the lightreceiving element through the InP substrate 110. As illustrated in FIG.7B, the pattern of the first back surface electrode 1187 is circular andthe diameter of the pattern is φ=200 μm. In the present embodiment,since the ohmic electrode is further disposed as the second back surfaceelectrode 1188, the whole bottom surface looks like an ohmic electrodeat first glance.

The first back surface electrode 1187 is composed of metal that containsbarrier metal, as with the above embodiments. On the bottom of n-typeInP that is the conductive semiconductor substrate 110, titanium,platinum and gold are attached in this order by evaporation. A thicknessof each of the layers is about 500 Å. Meanwhile, for the second backsurface electrode 1188, an alloy of germanium-containing gold and nickelis used. The alloy is deposited on the InP substrate 110 by evaporationand then heat-treated so that gold and germanium that contact InP arediffused into InP, thereby reducing a Schottky barrier and making theinterface ohmic.

Such a configuration, as with the above embodiments, suppressesinterdiffusion between InP and gold, thereby forming a good mirror-likethin film. In addition, since the first back surface electrode 1187 isnot disposed over the whole surface, an area of platinum, which tends togenerate stress due to heat change, can be reduced. Therefore, a thinfilm can be formed that maintains a mirror-like surface in spite of heatchange and is unlikely to be peeled off. Further, in this structure,since ohmic contact is formed in a region where the first back surfaceelectrode is not disposed, contact resistance can be reduced.

Operation of the optical semiconductor device according to the presentembodiment will be described. First, a reverse bias voltage is appliedbetween the front surface electrode 119 and the back surface electrode1187. As illustrated in FIG. 7A, most of the incident light 150 inputtedinto the light receiving section 140 through the insulating film 116from the surface is photoelectrically converted in the light absorbinglayer 112. However, part of the incident light 150 inputted into thelight absorbing layer 112 is not completely photoelectrically convertedin the light absorbing layer 112 and becomes the substrate-transmittinglight 152, which is reflected by the first back surface electrode 1187.Since the first back surface electrode 1187 is a mirror-like thin film,the back surface exhibits not diffuse reflection but reflectionoperation like specular reflection. Therefore, light that reaches anadjacent element 162 by diffuse reflection on a back surface as in theconventional example can be reduced, thereby reducing crosstalk.

An interface between InP and the back surface where the first backsurface electrode 1187 is not disposed becomes rough due to ohmicprocessing. However, there is little substrate-transmitting light 152that reaches the region, an effect of scattering can be ignored.

In the present embodiment, compared with the above embodiments, sincethe ohmic contact is formed in the region where the first back surfaceelectrode 1187 is not disposed, contact resistance can be reduced.According to current-voltage characteristics of a produced element, anincrease of contact resistance was within 1Ω, which indicates goodcharacteristics.

Next, a variation of the third embodiment will be described withreference to FIGS. 8A and 8B. FIGS. 8A and 8B are views illustrating aconfiguration of a semiconductor device, in which FIG. 8A is a crosssectional view including a light receiving section and FIG. 8B is abottom view.

This structure is different from that of the embodiment in FIGS. 7A and7B with respect to a pattern shape of the back surface. In the presentembodiment, in addition to the pattern (diameter: 200 μm) placed so thatan optical axis passing through the light receiving section 140 is inthe center of the pattern, back surface electrodes each having a smalldiameter (about 50 μm) are placed around the pattern.

In addition to effects of the invention in the embodiment illustrated inFIGS. 7A and 7B, in such a structure, tight adhesion between the backsurface electrodes having a small diameter and metal solder increasesadhesive strength between the optical semiconductor element and themetal solder, thereby enabling the optical semiconductor element tostrongly be fixed to the electric wiring and housing. As a result ofsubjecting a produced optical semiconductor element to delamination test(die shear test), it turns out that when pressing force is applied fromthe short side, a mean strength is 2 kgf for 3 samples. This isequivalent to a die shear strength of the conventional example.

An optical semiconductor device according to a fourth embodiment of thepresent invention will be described with reference to FIG. 9. FIG. 9 isa cross sectional view of the optical semiconductor device including alight receiving section.

As with the above embodiments, such an optical semiconductor device hasthe light absorbing layer 112 formed on the conductive semiconductorsubstrate 110 and has the plurality of diffusion regions 120 that have aconductive property opposite to that of the conductive semiconductorsubstrate 110. Description of configurations of the carrierconcentration, electrodes, and the like is left out since theconfigurations are the same as those of the above embodiments.

The present embodiment is structurally different from the aboveembodiments in that in the present embodiment, an insulating film 190 isdisposed as a mirror-like thin film. In the present embodiment, theinsulating film 190 using silicon nitride is deposited over the wholeback surface by a vapor phase growth method.

Operation of the optical semiconductor device according to the presentembodiment will be described. First, a reverse bias voltage is appliedbetween the front surface electrode 119 and the metal solder 130. Asillustrated in FIG. 9, most of the incident light 150 inputted into thelight receiving section 140 through the insulating film 116 from thesurface is photoelectrically converted in the light absorbing layer 112.However, part of the incident light 150 inputted into the lightabsorbing layer 112 is not completely photoelectrically converted in thelight absorbing layer 112 and becomes the substrate-transmitting light152, which transmits through the insulating film 190 and is reflected bythe metal solder 130 on the bottom of the insulating film 190. Since theinsulating film 190 and metal solder 130 are not so affected byinterdiffusion and the like, the metal solder 130 serves as amirror-like thin film and accordingly the back surface exhibits notdiffuse reflection but reflection operation like specular reflection.Therefore, light that reaches an adjacent element 162 by diffusereflection on the back surface as in the conventional example can bereduced, thereby reducing crosstalk.

Between the insulating film 190 and the metal solder 130 on the backsurface is electrically insulated. Accordingly, conduction between theconductive semiconductor substrate 110 and the metal solder 130 is madein such a way that the metal solder 130 goes around the insulating filmto contact to the side surface of the conductive semiconductor substrate110. Therefore, since the InP substrate 110 has a Schottky contact withthe metal solder 130, increase of contact resistance is concerned.However, by securing an area where the metal solder 130 contacts to theside surface by, for example, increasing an amount of the metal solder130, the increase of contact resistance often does not matter in qualityunder a normal operating condition.

Next, a variation of the fourth embodiment will be described withreference to FIG. 10. FIG. 10 is a cross sectional view of an opticalsemiconductor device including a light receiving section.

This structure is different from that of the embodiment illustrated inFIG. 9 in that in this structure, the back surface electrode 118 isfurther disposed on the bottom of the insulating film 190 disposed onthe back surface. In addition, by setting a thickness of the insulatingfilm 190 to about 0.2 μm, a high reflectance condition for the incidentlight is fulfilled. Since the insulating film 190 is disposed betweenthe back surface electrode 118 and InP, interdiffusion between the backsurface electrode 118 and InP does not occur. Therefore, the backsurface electrode 118 may be composed of metal that does not containbarrier metal, such as metal composed of only titanium and gold.

In addition to effects of the invention in the embodiment illustrated inFIG. 9, in such a structure, existence of the back surface electrodeenables a more stable mirror-like thin film to be formed than the thinfilm by metal solder. Since the insulating film fulfills a highreflectance condition, a reflectance is more improved than that of theembodiment in FIG. 9, thereby reducing crosstalk and improving lightreceiving sensitivity. If the back surface electrode does not containbarrier metal, stress due to heat change does not occur and therefore athin film can be formed that maintains a mirror-like surface in spite ofheat change and is unlikely to be peeled off.

An optical semiconductor device according to a fifth embodiment of thepresent invention will be described with reference to FIGS. 11A and 11B.FIGS. 11 A and 11B are views illustrating a configuration of the opticalsemiconductor device, in which FIG. 11A is a cross sectional viewincluding a light receiving section and FIG. 11B is a bottom view beforemetal-soldering.

As with the above embodiments, such an optical semiconductor device hasthe light absorbing layer 112 formed on the conductive semiconductorsubstrate 110 and has the plurality of diffusion regions 120 that have aconductive property opposite to that of the conductive semiconductorsubstrate 110. Description of configurations of the carrierconcentration, electrodes, and the like is left out since theconfigurations are the same as those of the above embodiments.

The present embodiment is structurally different from the aboveembodiment in that in the present embodiment, the insulating film 190 isdisposed as a mirror-like thin film and the insulating film 190 is notdisposed over the whole surface but patterned. The patterning isperformed in such a way that patterned metal is formed on the insulatingfilm 190 by a lift-off process and etching is carried out using themetal as a mask.

A position to be patterned is opposite to each light receiving sectionon the surface of the light receiving element through the InP substratefrom. That is, the pattern on the back surface is positioned so that anoptical axis passing through the light receiving section is in thecenter of the pattern. In the present embodiment, as illustrated in FIG.11B, the pattern of the electrodes is circular and a diameter of thepattern is p=200 μm. In a region without any pattern, the conductivesemiconductor substrate 110 is uncovered.

Operation of the optical semiconductor device according to the presentembodiment will be described. As with the above embodiment, first, areverse bias voltage is applied between the front surface electrode 119and the metal solder 130. As illustrated in FIG. 11A, most of theincident light 150 inputted into the light receiving section 140 throughthe insulating film 116 from the surface is photoelectrically convertedin the light absorbing layer 112. However, part of the incident light150 inputted into the light absorbing layer 112 is not completelyphotoelectrically converted in the light absorbing layer 112 and becomesthe substrate-transmitting light 152, which passes through theinsulating film 190 and is reflected by the metal solder 130 on thebottom of the insulating film 190. Since the insulating film 190 andmetal solder 130 are not so affected by interdiffusion, the metal solder130 serves as a mirror-like thin film and accordingly the back surfaceexhibits not diffuse reflection but reflection operation like specularreflection. Therefore, light that reaches an adjacent element 162 bydiffuse reflection on the back surface as in the conventional examplecan be reduced, thereby reducing crosstalk.

Between the insulating film 190 and the metal solder 130 on the backsurface is electrically insulated. Accordingly, conduction between theconductive semiconductor substrate 110 and the metal solder 130 is madein such a way that an uncovered region of the conductive semiconductorsubstrate 110 contacts to the metal solder 130 and the metal solder 130goes around to contact to the side surface of the conductivesemiconductor substrate 110. Therefore, since the InP substrate 110 hasa Schottky contact with the metal solder 130, increase of contactresistance is concerned. However, by making a contact area between theuncovered region of the conductive semiconductor substrate 110 and themetal solder 130 larger or by securing an area where the metal solder130 contacts to the side surface by, for example, increasing an amountof the metal solder 130, the increase of contact resistance often doesnot matter in quality under a normal operating condition.

Next, a variation of the fifth embodiment will be described withreference to FIGS. 12A and 12B. FIGS. 12A and 12B are views illustratinga configuration of the optical semiconductor device, in which FIG. 12Ais a cross sectional view including a light receiving section and FIG.12B is a bottom view before metal-soldering.

This structure is different from that of the embodiment illustrated inFIGS. 11A and 11B in that in this structure, the back surface electrode118 is further disposed on the bottom of the insulating film 190disposed on the back surface. In addition, a high reflectance conditionfor the incident light is fulfilled by setting a thickness of theinsulating film 190 to about 0.2 μm. Since the insulating film 190 isdisposed between the back surface electrode 118 and InP, interdiffusionbetween the back surface electrode 118 and InP does not occur.Therefore, the back surface electrode 118 may be composed of metal thatdoes not contain barrier metal, such as metal composed of only titaniumand gold.

Further, this structure is different from that of the embodimentillustrated in FIGS. 11A and 11B with respect to a pattern shape of theback surface. In the present embodiment, in addition to a pattern(diameter: 200 μm) placed so that an optical axis passing through thelight receiving section 140 is in the center of the pattern, backsurface electrodes, each having a small diameter (diameter: 50 μm), areplaced around the pattern.

In addition to effects of the invention in the embodiment illustrated inFIG. 10, in such a structure, tight adhesion between the back surfaceelectrodes having a small diameter and metal solder increases adhesivestrength between the optical semiconductor element and the metal solder,thereby enabling the optical semiconductor element to be strongly fixedto the electric wiring and housing. Since the insulating film 190electrically insulates between the back surface electrode 118 and theconductive semiconductor substrate 110, increase of an area of the backsurface electrode causes concern of increase of contact resistance.However, by making a contact area between an uncovered region of theconductive semiconductor substrate 110 and the metal solder 130 larger,or by securing an area where the metal solder 130 contact to the sidesurface by, for example, increasing an amount of the metal solder 130,the increase of contact resistance often does not matter in qualityunder a normal operating condition. If the back surface electrode doesnot use barrier metal, stress due to heat change does not occur andtherefore a thin film can be formed that maintains a mirror-like surfacein spite of heat change and is unlikely to be peeled off.

An optical semiconductor device according to a sixth embodiment of thepresent invention will be described with the use of FIGS. 13A and 13B.FIGS. 13A and 13B are views illustrating a configuration of the opticalsemiconductor device, in which FIG. 13A is a cross sectional viewincluding a light receiving section and FIG. 13B is a bottom view.

As with the above embodiments, such an optical semiconductor device hasthe light absorbing layer 112 formed on the conductive semiconductorsubstrate 110 and has the plurality of diffusion regions 120 that have aconductive property opposite to that of the conductive semiconductorsubstrate 110. Description of configurations of the carrierconcentration, electrodes, and the like is left out since theconfigurations are the same as those of the above embodiments.

The present embodiment is structurally different from the aboveembodiment in that in the present embodiment, the insulating film 190 isdisposed as a mirror-like thin film, and on the bottom of the insulatingfilm 190, an ohmic electrode is further disposed as the back surfaceelectrode 118 over the whole back surface. In the present embodiment,the insulating film 190 is patterned, not a whole surface, and on thebottom of the insulating film 190, the ohmic electrode is furtherdisposed as the back surface electrode 118 over the whole back surface.The method and structure of the patterning are the same as those of thefifth embodiment. That is, the patterning is performed by etching withthe use of a metal mask produced by a lift-off process and the positionof the pattern is opposite to each light receiving section on thesurface of the light receiving element through the InP substrate. Asillustrated in FIG. 13B, the pattern of the insulating film 190 iscircular and the diameter of the pattern is φ=200 μm. In the presentembodiment, since the ohmic electrode is further disposed, the wholebottom surface looks like an ohmic electrode at first glance.

An alloy of germanium-containing gold and nickel is used for the backsurface electrode 118 to be subjected to ohmic processing. The alloy isdeposited on the InP substrate 110 by evaporation and then heat-treated,thereby causing gold and germanium that contact to InP to be diffusedinto InP and therefore reducing a Schottky barrier and making theinterface ohmic.

Operation of the optical semiconductor device according to the presentembodiment will be described. First, a reverse bias voltage is appliedbetween the front surface electrode 119 and the back surface electrode118. As illustrated in FIG. 13A, most of the incident light 150 inputtedinto the light receiving section 140 through the insulating film 116from the surface is photoelectrically converted in the light absorbinglayer 112. However, part of the incident light 150 inputted into thelight absorbing layer 112 is not completely photoelectrically convertedin the light absorbing layer 112 and becomes the substrate-transmittinglight 152, which passes through the insulating film 190 and is reflectedby the back surface electrode 118 on the bottom of the insulating film.Since the insulating film 190 and back surface electrode 118 are not soaffected by interdiffusion, the back surface electrode 118 serves as amirror-like thin film and accordingly the back surface exhibits notdiffuse reflection but reflection operation like specular reflection.Therefore, light that reaches an adjacent element 162 by diffusereflection on the back surface as in the conventional example can bereduced, thereby reducing crosstalk. In addition, since ohmic contact isformed in a region where the insulating film 190 is disposed, contactresistance can be reduced.

An interface between InP and a region of the back surface where theinsulating film 190 is not disposed becomes rough. However, since thereis little substrate-transmitting light 152 that reaches the region, aneffect of scattering can be ignored.

Next, a variation of the sixth embodiment will be described withreference to FIGS. 14A and 14B. FIGS. 14A and 14B are views illustratinga configuration of a semiconductor device, in which FIG. 14A is a crosssectional view including a light receiving section and FIG. 14B is abottom view.

This structure is different from that of the embodiment illustrated inFIGS. 13A and 13B in that in this structure, the first back surfaceelectrode 1187 is further disposed on the bottom of the insulating film190 disposed on the back surface and further an ohmic electrode isdisposed as the second back surface electrode 1188. By setting athickness of the insulating film 190 to 0.2 μm, a high reflectancecondition for the incident light is fulfilled. Since the insulating film190 is disposed between the first back surface electrode 1187 and InP,interdiffusion between the first back surface electrode 1187 and InPdoes not occur. Therefore, the first back surface electrode 1187 may becomposed of metal that does not contain barrier metal, such as metalcomposed of only titanium and gold.

In addition, this structure is different from that of the embodimentillustrated in FIGS. 13A and 13B with respect to a pattern shape of theback surface. In the present embodiment, in addition to a pattern(diameter: 200 μm) placed such that an optical axis passing through thelight receiving section is in the center of the pattern, a plurality ofback surface electrodes, each having a small diameter (diameter: about50 μm), are placed around the pattern.

In addition to effects of the invention in the embodiment illustrated inFIGS. 13A and 13B, in such a structure, tight adhesion between the backsurface electrodes having a small diameter and the metal solderincreases adhesive strength between the optical semiconductor elementand the metal solder, thereby enabling the optical semiconductor elementto be strongly fixed to the electric wiring and housing. If the backsurface electrode does not use barrier metal, stress due to heat changedoes not occur and therefore a thin film can be formed that maintains amirror-like surface in spite of heat change and is unlikely to be peeledoff.

An optical semiconductor device according to a seventh embodiment of thepresent invention will be described with the use of FIGS. 15 to 17. Inthe present embodiment, optical semiconductor elements each in whichcrosstalk has been reduced using the above embodiment aretwo-dimensionally disposed.

FIG. 15 illustrates, as an example, an array of the opticalsemiconductor elements with 12 channels, in which light receivingelements are arranged in 4 rows×3 columns. The number of the elementscan be increased or decreased according to application.

As with the above embodiments, such an optical semiconductor device hasthe light absorbing layer 112 formed on the conductive semiconductorsubstrate 110 and has the plurality of diffusion regions 120 that have aconductive property opposite to that of the conductive semiconductorsubstrate 110. Description of configurations of the carrierconcentration, electrodes, and the like is left out since theconfigurations are the same as those of the above embodiments.

In this embodiment, as with the first embodiment, metal that containsbarrier metal is disposed as the back surface electrode 118 that is amirror-like thin film over the whole back surface.

In the configuration in FIG. 16, an array of the optical semiconductorelements arranged in 2 lines is accommodated in the housing 182 asillustrated. The optical semiconductor element 100 is accommodated inthe box-shape housing 182 made of ceramic. Since this housing 182 and awindow lid, which enables light to enter the light receiving section140, are bonded by metal solder (not shown), the optical semiconductorelement 100 can be protected from an external environment, be excellentin humidity resistance and have high reliability in a highly hermeticcondition. The optical semiconductor element 100 is accommodated in thehousing 182 in such a way that the light receiving section 140 faces thewindow lid, the back surface electrode and the housing 182 are fixed bythe metal solder 130 and the like, and the front surface electrode 119is connected by the bonding wire 132 to the electrical wiring 136 withinthe housing 182.

The electrical wiring 136 in the housing penetrates through the housing182 to the surface of the housing (not shown), allowing for electricalconnection to an electric wiring board or the like connected to outsidevia lead pins 138 fixed to the housing 182.

FIG. 17 shows a result of evaluation of an amount of crosstalk whenlight was inputted from an optical fiber. This evaluation was performedin such a way that the optical semiconductor element 100 was mounted onthe housing 182 in FIG. 16, and light was inputted into the lightreceiving section 140 with the use of optical fibers without sealing bythe window lid. Then, measurement was performed with the use of incidentlight having a wavelength of 1.55 μm in a room temperature environment.A crosstalk value of a vertical axis is a ratio of a light receivingcurrent in the light inputted element and a light receiving current inan adjacent element. In doing measurement, by changing a distance (z)between the light receiving section and an end face of the opticalfiber, a change of crosstalk was observed.

From the above experiment result, an adjacent crosstalk value in aconventional element is −35 dB to −42 dB in a region of z<600 μm,whereas an adjacent crosstalk value in the element of the presentinvention is −45 dB to −52 dB, that is, a reduction of 10 dB crosstalkcan be identified, as described above (see FIG. 4). Further, atwo-dimensional crosstalk has almost the same value as that of theadjacent crosstalk of the present invention, which shows a goodcrosstalk reduction effect.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. An optical semiconductor device comprising: aconductive semiconductor substrate; a light absorbing layer formed onthe conductive semiconductor substrate; a conductive semiconductor layerformed on the light absorbing layer, the conductive semiconductor layercomprising a plurality of diffusion layers that has a conductiveproperty opposite to a conductive property of the conductivesemiconductor substrate thereby to form light receiving elements in anarray, and a back surface electrode positioned on a bottom surface ofthe conductive semiconductor substrate, the back surface electrodecomprising a mirror-like thin film that reflects light specularly, themirror-like thin film comprising a barrier metal, a Schottky contactbeing formed between the conductive semiconductor substrate and the backsurface electrode.
 2. The optical semiconductor device according toclaim 1, wherein the mirror-like thin film is patterned.
 3. The opticalsemiconductor device according to claim 1, wherein the mirror-like thinfilm is patterned and a second back surface electrode that is an ohmicelectrode is formed on a bottom of the mirror-like thin film.
 4. Theoptical semiconductor device according to claim 1, wherein themirror-like thin film comprises an insulating film.
 5. The opticalsemiconductor device according to claim 4, wherein the mirror-like thinfilm comprises the insulating film and a back surface electrode on abottom of the insulating film.
 6. The optical semiconductor deviceaccording to claim 4, wherein the mirror-like thin film comprises theinsulating film and a back surface electrode on a bottom of theinsulating film, and is patterned.
 7. The optical semiconductor deviceaccording to claim 4, wherein the mirror-like thin film comprises theinsulating film and a first back surface electrode on a bottom of theinsulating film and is patterned, and a second back surface electrodethat is an ohmic electrode is formed on a bottom of the mirror-like thinfilm.
 8. The optical semiconductor device according to claim 1, whereinthe optical semiconductor device being accommodated in a housing.
 9. Theoptical semiconductor device according to claim 1, wherein the lightreceiving elements are two-dimensionally arranged.